Recent regulatory developments have led refiners to seek methods for reformulating motor fuels, including gasoline and diesel fuel, to meet increasingly stringent air quality requirements. These techniques include reducing the olefin and aromatic content of the motor fuels while maintaining the desired operational characteristics as predicted by the octane or cetane rating of the fuel.
Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate.
Industrial alkylation processes have historically used large volumes of liquid Bronsted acid catalysts such as hydrofluoric or sulfuric acid under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. Liquid acid catalyzed isoparaffin:olefin alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
The typical petroleum refinery generates numerous olefinic streams, which, upon hydrogenation and optional fractionation, would be useful gasoline blending components. Examples of such streams include the olefinic gasoline and naphtha by-products of catalytic hydrodewaxing processes such as the MLDW (Mobil Lubricant Dewaxing) and MDDW (Mobil Distillate Dewaxing). Additional examples include olefinic gasoline cuts from delayed coking units (thermally cracked gasoline), as well as from catalytic cracking process units such as a Fluidized Catalytic Cracking (FCC) process. Lighter olefins may be easily dimerized or oligomerized to provide suitable feedstocks, for example in a process such as MOGD/MOGDL (Mobil Olefins to Gasoline and Distillate/Mobil Olefins to Gasoline, Distillate and Lube Stock), or MOCI (Mobil Olefins to Chemical Intermediates). Examples of processes which product olefinic stocks include the processes taught in U.S. Pat. Nos. 4,922,048 to Harandi and 4,922,051 to Nemet-Mavrodin et al. Additional examples of light olefin dimerization/oligomerization processes include Dimersol (light olefin dimerization), Isopol (selective isobutene isomerization) and Selectopol (selective butadiene polymerization). See Hydrocarbon Processing, Vol. 61, No. 5, May 1982, pp. 110-112, and Hydrocarbon Processing, Vol. 60, No. 9, Sep. 1981, pp. 134-138.
Recent regulatory changes have created an incentive for refiners to reduce the olefins and aromatics content of motor fuels. The final version of the complex model issued by the United States Environmental Protection Agency (US EPA) to predict the consequence of various fuel components on combustion emissions creates a significant penalty for high RVP components in gasoline. At the same time, both the US EPA and state regulatory boards such as the California Air Resources Board (CARB) have instituted regulations on diesel fuel which set an upper limit on aromatics and sulfur contents, and a lower limit for cetane index. In general, sulfur must remain below 500 ppm. U.S. EPA requires either less than 35 wt % aromatics or a minimum of 40 cetane index. CARB limits aromatics to 10 wt % unless a waiver fuel is approved. Both regulatory agencies require a maximum T.sub.90 of 640.degree. F. By alkylating light olefins, such as C.sub.3 -C.sub.5 olefins, with light isoparaffins, such as isobutane and isopentane, high RVP gasoline components are converted into diesel range fuel which meets most of the regulatory restrictions.